Amorphous hydrogenated carbon hermetic structure and fabrication method

ABSTRACT

A hard layer of amorphous hydrogenated carbon (DLC) overlies a polymer film structure and a plurality of soft layers of DLC alternate with a plurality of hard layers of DLC over the barrier base to form a corrosion resistant structure. The polymer film structure and a circuit chip can be elements of a circuit module. The DLC and the polymer film structure can have vias extending to contact pads, and a pattern of electrical conductors can extend through the vias to the contact pads. In one embodiment the DLC forms a hermetic (and therefore corrosion resistant) seal over the polymer film structure.

The invention was made with Government support under contract numberF29601-92-C-0137 awarded by the Air Force. The Government has certainrights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates to hermetic coatings for multichip modulesand polymer film structures.

In one form of high density interconnect (HDI) circuit module, anadhesive-coated polymer film overlay having via openings covers asubstrate which can support integrated circuit chips in chip wells. Thepolymer film provides an insulated layer upon which is deposited ametallization pattern for interconnection of substrate metallizationand/or individual circuit chips through the vias. Methods for performingan HDI process using overlays are further described in Eichelberger etal., U.S. Pat. No. 4,783,695, issued Nov. 8, 1988, and in Eichelbergeret al., U.S. Pat. No. 4,933,042, issued Jun. 12, 1990. Generally aplurality of polymer film overlays and metallization patterns are used.

In another form of circuit module fabrication, as described by Cole etal., U.S. Pat. No. 5,527,741, issued Jun. 18, 1996, a method forfabricating a circuit module includes using a flexible interconnectlayer having patterned metallization on a base insulative layer and anouter insulative layer. At least one circuit chip having chip pads isattached to the base insulative layer and vias are formed in the outerand base insulative layers to expose selected portions of the baseinsulative layer metallization and the chip pads. A patterned outermetallization layer is applied over the outer insulative layer extendingthrough selected ones of the vias to interconnect selected ones of thechip pads and selected portions of the base insulative layermetallization.

Modules fabricated using the above processes have improved reliabilitywhen effectively sealed. Conventional hermetic sealing teachings includewelding, soldering, or glass-sealing a prefabricated hermetic lid to apackage including the module. These processes are not conformal to themodule and typically result in significant increases in packagingvolume. Furthermore, when dielectric materials such as glass or ceramicare used as sealing materials, electrical input/output connections mustbe formed prior to sealing.

Neugebauer et al., U.S. Pat. No. 5,336,928, issued Aug. 9, 1994,describes a conformal hermetic barrier fabricated by depositing acontinuous copper film and subsequently electroplating lead over thecopper. The resulting conformal barrier requires a continuous metallayer and a plurality of electrical connections cannot easily be madethough the hermetic layer.

SUMMARY OF THE INVENTION

Thus there is a particular need for an inexpensive conformal hermeticcoating fabrication process having (a) deposition conditions compatiblewith module materials (preferably at temperatures below 200° C.), (b)conformal deposition over non-planar topography; (c) depositions free ofpin holes; (d) appropriate dielectric properties; (e) patterningprocesses and materials compatible with module materials; (f) goodadhesion to underlying materials and any subsequently depositedmaterials; (g) resistance to damage from chemical attack or mechanicalabrasion; and (h) robustness in the presence of temperaturefluctuations. It would further be desirable to form a plurality ofelectrical connections through the hermetic seal without the loss ofhermeticity.

Amorphous hydrogenated carbon films, commonly referred to as diamondlike carbon (DLC) films, are used as dielectric materials and as scratchprotection coatings for various applications but have not been used toprovide hermetic barriers.

In the present invention, DLC is deposited in multiple hard and softlayers to form a composite DLC film possessing hermetic properties.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself, however,both as to organization and method of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description taken in conjunction with the accompanyingdrawings, where like numerals represent like components, in which:

FIG. 1 is a sectional side view of a module coated with a diamond likecarbon (DLC) composite film in accordance with the present invention.

FIG. 2 is a sectional side view wherein the DLC composite film surroundsthe module and further including a pattern of electrical conductorsextending through vias in the DLC composite film to contact pads of themodule.

FIG. 3 is a sectional side view showing a polymer film structure coatedwith a DLC composite film.

FIG. 4 is a graph illustrating the hermetic properties obtained withsamples of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 is a sectional side view of a module 6 having contact pads 13,15, and 17 and coated with a DLC composite film 8, and FIG. 2 is asectional side view wherein the DLC composite film surrounds the moduleand further including a pattern of electrical conductors 32 extendingthrough vias in the DLC composite film to contact pads 13, 15, and 17 ofthe module.

In one embodiment, the module 6 includes a substrate 10 and a polymerfilm structure 12, and the contact pads 13 include a metallization layerextending over and through (not shown) the polymer film structure tointerconnect chip pads 15 of circuit chips 19. Substrate 10 may compriseany suitable structural material such as a ceramic or a plastic, forexample. If a plastic is used and the DLC composite film is desired forforming a hermitic seal, then the DLC composite film preferablysurrounds the module, as shown in FIG. 2.

DLC composite film 8 provides a hermetic barrier, and therefore acorrosion resistant barrier. A "corrosion resistant barrier" as used inthe present invention refers to a barrier which is substantiallyresistant to corrosion in the environment for which its use is intended.For example, military applications use the standard JESD22-A110, HAST(highly accelerated stress test) 500 hours, 130° C., 85% RH (relativehumidity), and automotive applications use the standard JEDEC-STD22-A101at 1000 hours, 85° C., 85% RH.

DLC is deposited in multiple hard and soft layers by cycling the processpressure during deposition. Chemical vapor deposition can occur in astandard parallel plate plasma reactor at temperatures below 150° C. Inone embodiment, the DLC is deposited using an organic precursor whichincludes oxygen such as a methylethylketone (MEK) organic precursor.

Hard layers of DLC have better hermetic properties than soft layers ofDLC. DLC films, however, are deposited under high stresses, and hardfilms, in particular, typically can not be deposited at a thicknessabove about 1000 Å without cracking. By alternating the hard DLC layerswith lower stress, softer DLC layers, a conformal composite, pin-holefree film can be deposited to a thickness having several microns.

The hard or soft property of DLC can be obtained by controlling the DCbias voltage during film deposition. Soft DLC can be deposited in therange of about -100 volts to about -300 volts. At increasing magnitudesof bias voltage, the DLC becomes harder and more scratch resistant. Inone embodiment, the magnitude of bias voltage for hard DLC deposition isgreater than about -300 volts, and preferably the bias voltage magnitudeis in the range of about -450 volts to about -470 volts. In theindustry, the words "greater than" with respect to "bias voltage" areused to mean that the bias voltage has a higher magnitude (even though,because negative numbers are involved, a higher magnitude technicallyresults in a lower bias voltage). The phrase "bias voltage magnitude isgreater than" in the present invention description means that the biasvoltage has a higher magnitude and a corresponding more negative value.

The DC self bias voltage generated at the surface of the Rf poweredelectrode in a plasma chemical vapor deposition (CVD) reactor is ameasure of the amount of ion bombardment present. The bias voltage andthe amount of ion bombardment in a plasma discharge decreases as thechamber pressure is increased. As a result, DLC deposited at lowpressure is hard, and DLC deposited at high pressure is soft. The changein bias voltage can also be accomplished by changing the RF power level,but lowering the bias voltage by lowering the power results in lowdeposition rates.

A first layer 16 of the DLC composite film preferably comprises a hardlayer of DLC because hard layers have better adhesion properties thansoft layers. High pressure, soft DLC has good adhesion to the lowpressure, hard DLC, very good dielectric properties and lower leakage(amps per centimeters squared) as compared with hard DLC. A cap (outer)layer 20 of the DLC composite film preferably comprises a hard layer ofDLC because hard layers offer more scratch protection. Preferably, aplurality of layers of hard DLC are alternately applied with a pluralityof layers of soft DLC until a desired thickness is achieved, as shown inFIG. 2 with hard DLC layers 22, 26, and 20 and soft DLC layers 24 and28. A preferred thickness range of the DLC composite film is from about0.5 micrometers to about 2 micrometers. In one embodiment, thisthickness range is achieved with at least four layers of hard DLC and atleast 4 layers of soft DLC.

DLC composite film 8 forms a seal to substrate 10 and surrounds thematerial to be sealed. In the embodiment of FIG. 1, this material is thepolymer film structure 12. Polymer film structure 12 comprises amaterial on which DLC can be deposited. In one embodiment the polymerfilm structure comprises KAPTON™ polyimide (KAPTON is a trademark ofE.I. duPont de Nemours and Company). In a preferred embodiment, thematerial is KAPTON E polyimide, which has a low coefficient of thermalexpansion (about 17 parts per million per degree Celsius). If desired,in addition to sealing the polymer film structure, the DLC can bedeposited on all sides of module 6.

In order to make an electrical contact to contact pads, the DLCcomposite film 8 is patterned and etched. The etching of vias 30 can beperformed using an oxygen plasma mask process or an argon ion laserablation, for example. Vias can be etched to contact pads 13 on top ofoverlay structure 12. If desired, vias can be etched to chip pads 15 orsubstrate metallization 17.

After etching, the pattern of electrical conductors 32 can be depositedon the DLC composite film, in the vias, and on the contact pads. Thepattern of electrical conductors may comprise one or more layers ofelectrically conductive material. The layer adjacent the DLC compositefilm comprises a material capable of providing good adhesion to the DLCcomposite film such as titanium or tantalum, for example. In oneembodiment, the pattern of electrical conductors comprises a layer oftitanium having a thickness of 1000 Å, coated by a layer of copperhaving a thickness of 4 microns, coated by a layer of titanium having athickness of 1000 Å. In another embodiment, the pattern of electricalconductors comprises a layer of titanium having a thickness of 1000 Å,coated by a layer of copper having a thickness of 4 microns, coated by alayer of nickel having a thickness of 1 micron, coated by a layer ofgold having a thickness of 1500 Å. The pattern of electrical conductorsprovides a corrosion resistant seal to the DLC composite film and canprovide a hermetic barrier. The advantageous ability to make electricalconnections through a hermetic barrier of DLC is possible because theDLC is a dielectric material and not a conductive material. DLC issubstantially inert to chemical attack and can be patterned by usingoxygen plasma techniques or laser ablation.

In another embodiment, the DLC composite film can be used to prevent thecorrosion of flexible metal interconnections on polymer film structures.FIG. 3 is a sectional side view illustrating a free standing polymerfilm structure 312 coated on both sides with a polymer passivation layer311. The polymer film structure 312 can include metallization 313 on oneor both sides. DLC composite film 308 can be applied over the polymerpassivation layer 311 in a similar manner as discussed above withrespect to FIGS. 1 and 2.

EXAMPLE

DLC composite film was deposited on a 0.025 mm thick KAPTON E polyimidefilm using a methylethylketone (MEK) precursor for evaluation purposes.Four layers of hard DLC, each having a thickness of 450 Å, werealternated with four layers of soft DLC, each having a thickness of 3000Å. A cap layer of hard DLC was deposited to a thickness of 1000 Å. withthe total thickness of all the layers being 1.45 μm. During deposition,the pressure was automatically cycled using a microprocessor, and theelectrode temperature was maintained at 50° C. to reduce theredeposition of volatile organic species in the plasma.

Each of the four layers of hard DLC was deposited at a rate of 230 Å/minwith a flow of 15 sccm (standard cubic centimeters per minute), a powerof 50 W (watts) (DC bias -470 volts), and a pressure of 30 mtorr. Eachof the four layers of soft DLC was deposited at a rate of 300 Å/min witha flow of 15 sccm, a power of 50 W (DC bias -110 volts), and a pressureof 500 mtorr. The cap layer of hard DLC was deposited at a rate of 230Å/min with a flow of 15 sccm, a power of 50 W (DC bias -470 volts), anda pressure of 30 mtorr.

Hermeticity was tested by measuring helium penetration through the testfilms using a quadrapole mass spectrometer. The film was mounted in atwo cell vacuum vessel with the sample film separating upper and lowercells. After loading, both cells were evacuated and out-gassing wasmonitored in the lower cell until the sample stabilized at a pressureless than or equal to 1 millitorr. The baseline pressure was recordedusing a Baratron gauge, and the mass peak at 4 amu (atomic massunit-helium) was recorded using the mass spectrometer. Helium was thenintroduced to the upper cell, and the pressure was brought up to 7.3grams per square centimeter(g/cm²). The pressure differential across thesample membrane was approximately 14.7 g/cm².

Then the valve to the turbomolecular pump/mass spectrometer was closed,and the pressure change in the lower cell was monitored. Measurementswere taken every 60 seconds for 30 minutes. The valve to the massspectrometer was next briefly opened so that it could be verified thatthe primary species that penetrated the sample membrane was helium. Therate of the pressure rise in the test was assumed to be a measure ofhelium penetration.

FIG. 4 is a graph illustrating the pressure rise obtained with samplesdescribed above, a 0.025 mm thick sample of KAPTON E polyimide withoutany DLC composite film, and a 0.25 mm sample of a Kovar metal disk. Thepressure rise associated with the Kovar disk represents backgroundnoise. There is a substantial pressure rise from helium penetrationthrough the uncoated KAPTON E polyimide. The DLC coated polyimide,however, had a pressure rise substantially the same as that of the Kovardisk. Furthermore, no helium penetration was observed with the massspectrometer.

While only certain preferred features of the invention have beenillustrated and described herein, many modifications and changes willoccur to those skilled in the art. It is, therefore, to be understoodthat the appended claims are intended to cover all such modificationsand changes as fall within the true spirit of the invention.

What is claimed is:
 1. A hermetic structure comprising:a circuit moduleincluding a circuit chip and contact pads; a diamond like carbon (DLC)composite film including a barrier base comprising a hard layer of DLCover the circuit module and a plurality of soft layers of DLCalternating with a plurality of hard layers of DLC over the barrierbase, the DLC composite film having vias therein extending to thecontact pads; and a pattern of electrical conductors extending throughthe vias to the contact pads and providing a corrosion resistant seal tothe DLC composite film, the DLC composite film and the pattern ofelectrical conductors providing a hermetic barrier for the circuit chinand contact pads.
 2. The structure of claim 1 wherein the pattern ofelectrical conductors comprises a multilayer pattern of electricalconductors with a first conductive layer in contact with the DLCcomposite film and the contact pads and comprising an adhesion promotingconductive material.
 3. The structure of claim 2 wherein the firstconductive layer is titanium or tantalum.
 4. The structure of claim 2wherein the first conductive layer comprises titanium and the pattern ofelectrical conductors includes a second conductive layer overlying thefirst conductive layer and comprising copper.
 5. The structure of claim4 wherein the pattern of electrical conductors includes a thirdconductive layer overlying the second conductive layer and comprisingtitanium or gold.
 6. The structure of claim 1 wherein the circuit moduleincludes a polymer film structure over the circuit chip and wherein theDLC composite film forms a hermetic seal over the polymer film structureof the circuit module.
 7. The structure of claim 6 wherein the polymerfilm structure includes metallization thereon and the contact padscomprise portions of the metallization.
 8. The structure of claim 1,wherein the circuit module includes a substrate supporting the circuitchip.
 9. The structure of claim 8, wherein the substrate comprisesplastic and wherein the DLC composite film surrounds the circuit module.10. The structure of claim 8, wherein the substrate includesmetallization thereon and the contact pads comprise portions of themetallization.
 11. The structure of claim 1, wherein the circuit chipincludes chip pads and the contact pads comprise the chip pads.
 12. Ahermetic structure comprising:a circuit module including a circuit chipand contact pads; a diamond like carbon (DLC) composite film including abarrier base comprising a hard layer of DLC over the circuit module anda plurality of soft layers of DLC alternating with a plurality of hardlayers of DLC over the barrier base, the DLC composite film having viastherein extending to the contact pads; and a pattern of electricalconductors extending through the vias to the contact pads and providinga corrosion resistant seal to the DLC composite film, the pattern ofelectrical conductors comprising a multilayer pattern of electricalconductors with a first conductive layer in contact with the DLCcomposite film and the contact pads and comprising an adhesion promotingconductive material, the DLC composite film and the pattern ofelectrical conductors providing a hermetic barrier for the circuit chipand contact pads.